Functional programming in Python

Functional Programming in Python
(PYTHON)( )

TODAY'S SLIDES
http://goo.gl/iRXD0Y

(PYTHON)( )
ABOUT ME
+ Colin Su a.k.a LittleQ
+ http://about.me/littleq
+ Productional Experience:
- Erlang
- Python
- Java{,Script}

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PROGRAMMING IN PYTHON
+ Imperative Programming
- Shell Scripting
+ Procedural Programming
- Languages with functions
+ Declarative Programming
- Functional Programming

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FUNCTIONAL PROGRAMMING
+ No state
+ Immutable data
+ Function as ﬁrst-class citizen
+ Higher-order function
+ Purity
+ Recursion, tail recursion
+ ...the list is too long to name here

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IP VS FP
$ ./program1
$ ./program2 --arg=1
$ ./program3
$ ./program1 | ./program2 --arg=1 | ./program3
Imperative Functional
Your Terminal

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FUNCTION AS FIRST CLASS OBJ
def add(a, b):
return a + b
add2 = add
add2(1,2) # 3
def giveMeAdd():
def add(a, b):
return a + b
return add

(PYTHON)( )
LAMBDA SUPPORT
add = lambda a, b : a + b
minus = lambda a, b : a - b
multiply = lambda a, b : a * b
divide = lambda a, b : a / b

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(POOR) LAMBDA SUPPORT
add = lambda a, b:
c = a + b
return c

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(POOR) LAMBDA SUPPORT
add = lambda a, b:
c = a + b
return c
File "test.py", line 29
add = lambda a, b:
^
SyntaxError: invalid syntax

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HIGHER-ORDER FUNCTION
+ Function eats functions, and returns functions
def food():
return "food got eaten"
drink = lambda: "drink got drunk"
def me(*args):
def eat():
return map(lambda x: x(), args)
return eat
act = me(food, drink)
print act()
>>> ['food got eaten', 'drink got drunk']

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MAP/REDUCE/FILTER
+ A war between readability and conciseness
+ To make your code pluggable

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CALCULATE "1++2+3+++4+5"

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IMPERATIVE.PY
INPUT = "1+2++3+++4++5+6+7++8+9++10"
result = 0
for num in INPUT.split('+'):
if num:
result += int(num)
print result
Iterate on the same variable to perform task

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FUNCTIONAL.PY
from operator import add
INPUT = "1+2++3+++4++5+6+7++8+9++10"
print reduce(add, map(int, filter(bool, INPUT.split('+'))))

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FUNCTIONAL PYTHON
MADE SIMPLE

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BUT NOT ALL LANGUAGES
DO...

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IMPERATIVE.JAVA
Multiset<Integer> lengths = HashMultiset.create();
for (String string : strings) {
if (CharMatcher.JAVA_UPPER_CASE.matchesAllOf(string)) {
lengths.add(string.length());
}
}
Looks making sense

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FUNCTIONAL.JAVA
Function<String, Integer> lengthFunction = new Function<String, Integer>() {
public Integer apply(String string) {
return string.length();
}
};
Predicate<String> allCaps = new Predicate<String>() {
public boolean apply(String string) {
return CharMatcher.JAVA_UPPER_CASE.matchesAllOf(string);
}
};
Multiset<Integer> lengths = HashMultiset.create(
Iterables.transform(Iterables.filter(strings, allCaps), lengthFunction));

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Welcome To Functional

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But my typing speed really got increased since using Java.

PARTIAL FUNCTION
APPLICATION
(((a × b) → c) × a) → (b → c)

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PARTIAL FUNCTION
APPLICATION
from functools import partial
def f(x, y, z):
return x + y + z
f2 = partial(f, 100, 10)
print f2(5) #115

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PARTIAL FUNCTION
APPLICATION
+ Keyword-based partial application
from datetime import datetime
def log(message, prefix="", postfix=""):
print prefix, message, postfix
error = partial(log, postfix=datetime.now(), prefix="[ERROR]")
error("something goes wrong")
# [ERROR] something goes wrong 2014-04-10 01:37:07.250509

CURRYING
((a × b × c) → d) → (((a → b) → c) → d)

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CURRYING
+ Simple sum
def simple_sum(a, b):
return sum(range(a, b+1))
>>> simple_sum(1, 10)
55

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CURRYING
+ Squared sum
def square_sum(a, b):
return sum(map(lambda x: x**2, range(a,b+1)))
>>> square_sum(1,10)
385

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CURRYING
+ Square root sum
def sqrt_sum(a, b):
return sum(map(math.sqrt, range(a,b+1)))
>>> sqrt_sum(1,10)
22.4682781862041

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CURRYING
+ Curried sum()
import math
def fsum(f):
def apply(a, b):
return sum(map(f, range(a,b+1)))
return apply
simple_sum = fsum(int)
square_sum = fsum(lambda x: x**2)
sqrt_sum = fsum(math.sqrt)
print simple_sum(1,10) # 55
print square_sum(1,10) # 385
print sqrt_sum(1,10) # 22.4682781862

(PYTHON)( )
CURRYING
+ Combined with partial
def fsum(f):
def apply(a, b):
return apply
from operator import mul
mul_sum = fsum(partial(mul, 2))
print mul_sum(1,10) # 110

(PYTHON)( )
DECORATOR
+ Another way to curry
import math
def fsum(f):
def apply(a, b):
return apply
@fsum
def sqrt_sum(x):
return math.sqrt(x)
print sqrt_sum(1,10) # 22.4682781862

(PYTHON)( )
GENERIC CURRY IN PYTHON
import functools
def curry(func):
def curried(*args, **kwargs):
if not args and not kwargs:
return func()
return curry(functools.partial(func, *args, **kwargs))
return curried
@curry
def add(a, b, c, d, e, f, g):
return a + b + c + d + e + f + g
add12 = add(1)(2)
add1234 = add12(3)(4)
add1234567 = add1234(5)(6)(7)
print add1234567() # 28

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TAIL RECURSION
+ No grammar-level support, just simulation
def printAll(strings):
if strings:
print strings[0]
return printAll(strings[1:])
printAll([1,2,3,4,5])
Python 2
def printAll(strings):
if strings:
head, *tail = strings
print(head)
return printAll(tail)
printAll([1,2,3,4,5])
Python 3

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FUNCTIONAL PYTHON?
Pros
+ First-class function
+ lambda
+ built-in map/ﬁlter/reduce
+ functools
+ generators as lazy-evaluation
Cons
+ non-pure
+ lambda (?)
+ memory-cost operations
+ No optimization for tail recursion
+ No pattern matching

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CONCLUSION
+ Python is not for replacing any functional
language
+ But a good way to begin functional
programming

(PYTHON)( )
ALL CODE SAMPLES ON GIST

Functional programming in Python

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Functional programming in Python